US2269972A - Control system for synchronous machines - Google Patents

Description

Patented-Jan-'1 9 2 Mayncrd N. new, South, N. Y., asslgnor to General Electric Company, a

New York corporation of Application September 23, 1939, Serial No. 296,271

' 17 Claims. (Cl. 172-289) My invention relates to control systems for synchronous machines and particularly to a system for efiectingpredetermined changes in the connections of a subsynchronously operating synchronous machine at predetermined points in i the slipcycle when it is operating at a predetermined subsynchronous speed.

'One object of my invention is to provide an improved arrangement of apparatus which can be easily adjustedso as to change the minimum subsynchronous speed at which the desired connection changes can be effected without materially changing the range in the slip cycle at which the connection changes are effected.

My invention will be better understood from' the following description when taken in connection with the accompanying drawing and 'its scope will be pointed ,out in the appended claims.

7 In the accompanying drawing Fig. 1' diagrammatically illustrates a synchronous motor excitation control system embodying my invention and Figs.2 and 3 are explanatory diagrams.

Referring to Fig. l of the accompanying drawing, l represents a synchronous motor having an armature winding and a field winding 3'. In order to simplify the disclosure, 1 have shown my invention in connection with a full voltage starting system whereby the motor is started as an induction motor by connecting the motor armature winding 2 directly to an alternating current supply. circuit it while the field winding 3 is shortcircuited'through a discharge resistor 5. As-

shown, the armature winding 2 is arranged to be connected [directly to the alternating current circuit 5 by means of a switch 6 having a closing coil 1 that is arranged to be energized fromone phase of the supply circuit 4 when a control relay 8 is energized. The control relay 8, in turn, is arranged to be energized in response to the closing of a control switch 8 which is shown as a manually operated device, but it will be obvious to those skilled in the art that it may be automatically controlled in any suitable manner so that it is closed when it is desired tostart the motor i. In the energizing circuit of the control relay 8, I also provide the contacts of a normally closed switch H) which is controlled in any suitable manner so that its contacts are opened when it is desired to stop the motor 1. The switch 6, when closed, completes through its auxiliary'contacts H, locking circuits for the closing coil 1 and the winding of control relay 8. 4

The field winding 3 of the motor I is arrange to be connected to a suitable source of excitation I2 by means of a field switch l3 which may be of any suitable construction. As shown, the field switch has a-holding winding M which, when energized, tends to maintain the field switch in its open position, which is the position in which it is shown, and a closing winding 15 which, when energized, tends to move the switch to its closed position. The switch I3 is so constructed that the winding I5, can move the switch to its closed position only after the winding H has been deenergized for a predetermined time. A resistor E6 of the proper resistance value is connected in parallel with the holding winding [4 for obtaining this predetermined time delay. When the field switch I3 is in its open position, its auxiliary.

contacts I! connect the discharge resistor 5 across the terminals of the field winding 3.

For controlling the closing and opening of the field switch I3, I have shown a motor armature impedance relay arrangement of the type disclosed and claimed in United States Letters Pat- 'ent 2,151,160' granted March 21, 1939, to the. as-

signee of this application on an application by Harold T. Seeley. Asshown, this relay arrangement comprises an impedance relay I8 having a rotatable member l9 and a cooperating wattmetric driving element 20 that includes a current winding 2| energized in response to the current in one of the phase conductors supplying current to the motor armature winding 2 and a voltage winding 22 connected in series with a capacitor 23 and a resistor 24 across one of the phases of the supply circuit 4. The driving element 20 therefore exerts a torque which is a function of the supply circuit voltage .and the motor armature current. The rotatable element [9 is also provided with another driving element 25 which has a voltage winding 26 connected in series with a capacitor and the resistors 6| and 62 toone phase of the supply circuit through the contacts of switch l0 and auxiliary contacts of the circuit breaker 6 when it .is closed. The driving element 25 exerts a torque proportional to the square of supply circuit voltage in a direction to open contacts 28 so that the operation of the relay IB varies with the impedance of the motor armature circuit. The voltage restraining torque exerted by the driving element 25 may, be varied by means of a suitable potentiometer 29 assbci ated with the winding 26.

By varying the relative phases of the currents in the windings 2| and 22 of the relay It in any well known manner, as for example by varying the electrical constants of the circuit of one or both of the windings 2| and 22, the zero torque characteristic of the relay, which is substantialiy a straight line, can be made to have any desired slope, and by adjusting the potentiometer 2! so as to vary the torque exerted by the driving element 25, the distance of this zero torque characteristic from the origin can be adjusted to any desired value. It the relay I8 is adjusted so that it has a zero torque characteristic, represented by the line MM in Fig. 2, the relay l8 maintains its contacts 23 closed and its contacts 30 open whenever the magnitude and phase angle of the motor armature current is such that the current vector terminates at the right of the line MM and opens its contacts 28 and closes its contacts 30. whenever the magnitude and phase angle of the motor armature current is such that the current vector terminates at the left of the line MM. It will be seen that with this relay characteristic the motor speed has to increase to nearly 96% of synchronous speed before the armature current pulsations are of such a character that the relay i8 opens its contacts 28 and closes its contacts 30 during any portion of a current pulsation cycle. In this Fig. 2 the various circles represent the locioi the ends of the motor armature current vectors at a number of speeds near synchronous speed. From this ilgure it will be seen that up to a predetermined speed, the motor armature current vector terminates continu ously at the right of the zero torque characteristic MM. As the motor speed increases above this predetermined value. the armature current vecof the zero torque characteristic MM during,

each half cycle of slip when the motor speed is above said predetermined value increases very rapidly tor a small change in slip. For example,

it is at least longer when the motor is operating at 2% slip than at 3% slip.

By having the opening 0! the contacts 22-0! relay l3 initiate the operation 0! a definite time relay, the timing operation of which is such that when the motor I is operating at the desired minimum synchronizing speed, the timing operation is completed just as the contacts 23 reclose, this time relay will complete its timing operation at either of two definite points in the slip cycle which are substantially 180 or a halt cycle apart, when the motor is operating at the desired minimum synchronizing speed. The rea son there are two points, substantially 180 apart, in each slip cycle is because there is a pulsation in the armature current each time the unexcited salient field poles of the motor slip one pole pitch with respect to the rotating armature field poles.

In the embodiment of my invention shown in the drawing, the relay It has associated therecult of the winding 33 also includes, in series. the normally closed contacts 28 of relay l8, and the normally closed contacts 83 of the relay 32. Until the contacts 2| are opened continuously for a predetermined time interval, which depends upon the size of the condenser 31 and resistor 36, the holding winding 33 prevents the ener gized operating winding 34 from moving the relay 32 to its closed position. When the relay 32 moves .to its closed position, it changes the connections of the holding winding l4 0! the field switch 13 so that the contacts 23 of relay II are connected in series therewith. Therefore, the circuits to the holding windings of the time relay 32 and the field switch 13 are opened at the same points in the half slip cycle when the motor' is operating at the minimum speed at which it will eii'ect the operation of the time relay 32. This is an important feature of my present invention.

I have found from tests that if field excitation is applied near the point of minimum relay torque, that is, near the mid point of the arc of the circle in Fig. 2 during which time the contacts 28 are open, when the motor is operating at the minimum speed at which it will effect the operation of the relay 32 and the zero torque characteristic MM has a slope of 45", one of the two points in the slip cycle at which the field excitation is applied is near the best point in the slip cycle to apply excitation and the other point, which is displaced substantially 180 from the best point. is also a very good point to apply excitation compared to the worst point in the slip cycle. This will be seen more clearly from Fig. 3 which shows the per unit load torque that can be synchronized with excitation voltage applied at various points in the slip cycle. If the excitation is applied at point X, which is the best point in the slip cycle and at point Y, which is displaced 180 from point X, the synchronizing torque developed by the motor when excitation is applied at either of these two points is materially greater than when it is applied at the worst point Z in the slip cycle.

The operation of the embodiment of my invention shown inFig. 1 is as follows: When it is desired to start the motor I, the control switch 8 is closed to complete through the normally closed contacts oi the control switch Ill and the contacts 40 of the field switch l3, an energizing circuit for the control relay 8 across one phase of the supply circuit 4.

By closing its contacts 4! and 41, the control relay 8 completes an energizing circuit for the closing coil I of the switch 6 across one phase 32 is in its normally open position, energizing i connected resistor 43 and capacitor 44. The cirof the supply circuit 4. ing coil I also includes the contacts of the con trol switches 9 and ill. By closing its auxiliary contacts H, the switch 6 completes a shunt circuit around the contacts of the control switch 8 and the contacts 4| of relay 8 in the energizing circuit of the closing coil 1. The contacts II also complete a locking circuit for relay 6 through its contacts 4|.

By closing its auxiliary contacts 53, the switch 8 completes energizing circuits for the voltage windings 22 and 2B of the relay i8 across one phase of the supply circuit 4. The circuit oi. the

voltage winding 22 also includes the series connected resistor 24 and capacitor 23, which are shunted by the contacts 64 of the relay 8 when it is energized. The circuit of the voltage wind ing 28 includes the potentiometer 29 and the capacity and the resistors 6i and 62, the latter The circuit of the clos also includes the rectifier of which isshunted by the contacts 33 of relay 3 when it is energized. The closing of the contacts 33 of the switch Ii also completes energizingcircuits for the windings 33 and 34 of the time relay 32 and the windings I4 and I5 of the field switch I3 across one phase of the supply circuit 4. The circuit of the holding winding 33 which is permanently shunted by the series connected resistor 35 and capacitor 31 also includes the rectifier 35, contacts 42 of field switch I3, contacts 33 of relay 32, and contacts 23 of relay I3. The circuit of the operating winding 34, which is permanently shunted by the series connected resistor 43 and capacitor 44, also includes the rectifier 35, and the contacts 42 of the field switch I3. The circuit of the holding winding I4 of the field switch I3 and of the resistor I5, which is permanently connected in parallel with the winding I4, 35, contacts 42 of the field switch I3, and the contacts 45 of the time relay 32. The circuit of the operating winding I5 of the field switch I3 also includes the rectifier 35, and the contacts 45 of the relay 3. Although the operating windings I5 and 34 are energized, they cannot effect the operation of the field switch I3 and the time relay 32 respectively until after their respective holding windings have been deenergized for a predetermined time.

The closing of the contacts 33 of switch 5 also connects a resistor 55 in parallel with the winding I5 and contact 46 of relay 3 so as to give a higher average voltage across the winding I5 from the rectifier 35.

The closing of the main contacts of switch 3 connects the armature winding 2 of the motor I directly across the supply circuit 4 so that the motor I starts andaccelerates as an induction motor. During the starting operation, the motor field winding 3 is connected in circuit with the discharge resistor 5 by the contacts I] of the field switch I3. i

From Fig. 2 it will be seen that, when the motor I is operating at a high slip, phase of the motor armature current are such that the loci of the ends of the armature cur- 'rent vector are entirely to the right of the zero torque characteristic MM so that the relay I3 maintains its contacts 23 closed continuously and it is not until the motor reaches a speed near synchronous speed that the locus of the end of the armature current vector crosses the zero torque characteristic MM and causes the relay point in the slip cycle.

the magnitude and 1 I3 to open its contacts 23 during a portion of each half slip cycle. The opening of the contacts 23 interrupts the energizing circuit of the holding winding 33 of relay 32, but due to the series connected resistor 33 and capacitor 31 around the winding 33, the operating winding 34 does not eifect the operationof the relay 32 until the energizing circuit of the holding winding 33 remains continuously interrupted for a predetermined time.

When the moto l reaches the desired minimum synchronizing speed, which is usually the maximum speed to which the motor can acceler-- ate its maximum starting load, the contacts 23 remain open just long enough during each half slip cycle to effect the operation of the relay 32. By opening its contacts 33 in the circuit of the holding winding 33, the relay 32 insures that it will remain in its operated position when the contacts 23 reclose. By opening its contacts 45 and closing its contacts 43, the relay 32 transfers the connections of the holding winding I4 of the field switch I3 so that the contacts 23 are con- .the time delay of the relay nected in series therewith. Therefore, when the contacts 23 are open during the nexthalf slip cycle, the circuit of the holding winding I4 is opened for a sufilcient length of time to allow the operating winding I5 to close the fieldswitch I3. Forexample, if in Fig. 2 the arc CBA represents the length of time during each half slip cycle the contacts 23 are open when the motor is operating at a 3% slip and this length of time is just equal to the time delay of relay 32,'the relay 32 will operate at point A which is a definite If the total operating I3 is made equal to half 32, then during the next half cycle of slip the field switch I3 closes at point B which is the mid point of the arc CBA and which is either near the best point in the slip cycle to apply excitation or from the best point.

By opening its auxiliary contacts I1 and closing its main contacts 53 and 5I the field switch I3 disconnects the field winding 3 from the discharge resistor 5 and connects the field winding 3 to the source of excitation I2. By opening its auxiliary contacts 42, the field switch I3 interrupts the circuits of the holding winding I4 of the field switch I3 and the operating winding 33 of the time relay 32. After a time delay, determined by the resistor 43 and capacitor 44, the time relay 32 opens its contacts 43 and 53.

By opening its contacts 43, the field switch I3 effects the deenergization of the control relay 3, which by opening its contact 43 interrupts the original energizing circuit of the operating winding I5 of the field switch I3. After the field switch I3 closes and until the time relay 32 opens, an energizing circuit is maintained for the. operating winding I5 through the contacts 53 of the relay 32. By the time the relay 32 operates to time of the field switch open its contacts 53, which time is made long By opening its contacts 33, the control relay 3 removes the short circuit from around the resistor 32 in the energizing circuit of the voltage winding 25 of the relay I3 and by closing its contacts 34 completes a shunt circuit around the capacitor 23 and resistor 24 in the energizing circult of the winding 22 so that the zero torque characteristic of the relay is changed from MM to NN'. The opening of the contacts 41 of the relay 3 interrupts the original energizing circuit for the closing coil 1 of switch 3 but this coil does not become deenergized at this time because the auxiliary contacts 55 of field switch I3. which are in parallel with the contacts As long as the motor I remains in synchronism, the magnitude and phase angle of the armature current are such that the current vector terminates on. the left-hand side of the zero torque so that the contacts 33 of relay I3 remain closed. When, however, the motor I falls out of synchronism, the motor ar mature current vector terminates on the righthand side of the zero torque characteristic NN and the relay I3 opens its contacts 33 in the circult of the operating winding 15 of the field switch I3. The field 41, are closed the source of excitation the potentiometer 28 which changes the distancecf the zero torque characteristic MM in Fig. 2 from its origin without materially changing the points in the slip cycle at which the field excl tation is applied when the motor is operating at the new minimum synchronizing speed. This will be apparentirom Fig. 2 in which the line PP represents the zero torque characteristic of the relay II when the potentiometer has been ad- Justed so the minimum speed at which the field excitation can be applied is 98% synchronous speed. When the motor is operating at this minimum synchronizing speed, the contacts 28 open at point C and close at point A during each half cycle pulsation of armature current. Since the operating time or the field switch I3 is equal to halt the operating time or relay 32, the field switch l3 closes at point B which is the midpoint of the arc C'BA'. Since the point B is near the point of minimum relay torque, the field excitation is applied at a point near the best point in the slip cycle or 180 therefrom when the motor is operating at the new minimum synchronizing speed.

Therefore, after the operating times of the time relay 32 and the field switch 13 have been and phase range of the armature current or said machinerelative to the armature voltage oi. said machine, a second tirnlng device, meanscontrolled by said first timing device and said control means Ior effecting the timing operation or said second timing device only during said predetermined magnitude and phase range oi armature current after said first timing device has completed its timing operation, and means for controlling said control means so as to vary the predetermined range of armature current during which said control means can effect the operation oi said timing means.

3. In a system including an alternating current supply circuit connected to a synchronous ma chine, a control arrangement including timing device having a definite operating time, control means for eilectingthe operation of said "timing means only duringa predetermined magnitude and phase range of the armature current of said machinerelative to the armature voltage oi. said machine, a second timing device having an operating time less than the operating time of said first timing means, and means controlled by said first timing device and said control means 101' effecting the timing operation of said second timing device only during said predetermined magnitude and phase range of armature current properly adjusted relatively to each other to ap- ,ply the field excitation at proper points in the slip cycle for a given minimum synchronizing speed, no further adjustment of these operating times is required when the minimum synchronizing speed is changed by adjusting the potentiometer 29.

While I have, in accordance with the patent statutes, shown and described my invention as applied to a particular system and as embodying-various devices diagrammatically indicated, changes and modifications will beobvious to those skilled in the art, and I therefore aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In a system including an alternating current supply circuit connected to a synchronous machine, a control arrangement including a timing device having a definite operating time, control means for efiectlng the operation or said timing means only during a predetermined magnitude and phase range of the armature current of said machine relative to the armature voltage of said machine, a second timing device, and means con- 'trolled by said first timing device and said control means for effecting the timing operation or said second timing device only during said predetermined magnitude and phase range of armature current after said first timing device has completed its timing operation.

2. In a system including an alternating current supply circuit connected to a synchronous machine, a control arrangement including a timing device having a definite operating time, control means for effecting the operation of said timing means only during a predetermined magnitude after said first timing device has completed its timing operation.

4. In a system including an alternating current supply circuit connected to a synchronous machine, a control arrangement including a timing device having a definite operating time, control means for effecting the operation of said timing means only during a predetermined magnltucie and phase range of the armature current of said machine relative to the armature voltage of said machine, a second timing device having an operating time lessthan the operating time of said first timing means, means controlled by said first timing device and said control means for eilecting the timing operation of said second timing device only during said predetermined magnitude and phase range of armature current after said :llrst timing device has completed its timing operation, and means for controlling said control means so as to vary the predetermined range of armature current during which said control means can eiiect the operation of said timing means.

, 5. In a system including an alternating current supply circuit connected to a synchronous machine, a control arrangement includinga timing device having a definite operating time, control means for efiecting the operation of said timing means only during a predetermined magnitude and phase range of the armature current of said machine relative to the armature voltage of said machine, a second timing device having an operating time substantially equal to half the operating time of said first timing means, and means controlled by said first timing device and said control means for effecting the timing operation of said second timing device only during said predetermined magnitude and phase range of armature current after said first timing device has completed its timing operation.

6. In a system including an alternating current supply circuit connected to a synchronous machine, a control arrangement including a timing device having a definite operating time, control means for effecting the operation of said timing means only during a predetermined magnitude and phase range 01 the armature current of said machine relative to the armaturevoltage of said machine, a second timing device having an operating time substantially equal to half the operating time of said'first'timing means,

means controlled by said first timing device and said control means for effecting the timing operation of said second timing device only during said predetermined magnitude and phase range of armature current after said first timing device has completed its timing operation, and means for controlling said control means so as to vary the predetermined range of armature current during which. said control means can efiect the operation of said timing means.

, 7. In a system including an alternating current supply circuit connected to a synchronous machine, a control arrangement including atiming edvice having a definite operating time, control means for efiecting the operation of said said control means for effecting the timing operation of said second timing device only during said predetermined magnitude and phase range of armature current after said first timing device has completed its timing operation, and means responsive to said second timing means completing its timing operation for effecting a predetermined change in the connections of said machine.

8. In a system including an alternating current supply circuit connected to a synchronous machine, a control arrangement including a tim ing device having a definite operating time, control means for effecting the operation of said timing means only during a predetermined magnitude and phase range of the armature current cfsaid machine relative to the armature voltage of said machine, a second timing device having an operating time less than the operating time of said first timing means, means controlled by said first timing device and said control means for eflecting the timing operation of said second timing device only during said predetermined magnitude and phase range of armature current rent supply circuit connected to a synchronous 1 machine, a control arrangement including a timing device having a definite operating time, control means for effecting the operation of said timing means only during a predetermined magnitude and phase range of the armature current of said machine relative tothe armature voltage of said machine, a second timing device,

Ymeans controlled by said first timing device and said control means for effecting the timing op- I eration of said second timing device only during said predetermined magnitude and phase range of armature current after said first timing device has completed its timing operation, 'and means responsive to said second timing means completing itstiming operation for effecting a change in the field connections of said machine.

11. In a system including analternating current supply circuit connected to a synchronous machine, a control arrangement including a timing device havinga definite operating time, control means for efiecting the operation of said timing means only during a predetermined magnitude and phase range-of the armature current 01 said machine r'elative to the armature voltage of said machine, a. second timing device having an operating time less than the operating time of said first timing means, means controlled bysaid first timing device and said control means for effecting the timing operation of said second timing deviceo nly-during said predetermined magnitude and phase range of armature current after, said first timing device has completed its timing operation, and means responsive to said second timing means completing its timing operation for eflecting a change in the field connections of said machine.

12. In a system including an alternating current supply circuit connected to a synchronous machine, a control arrangementincludina a timingdevice having a definite operating time, control means for eflecting the operation of said timing means only during a predetermined ma nitude and phase range of the armature current after said first timing device has completed its eration for effecting a predetermined change in the connections of said machine.

9. In a system including an alternating current supply circuit connected to a synchronous machine, a control arrangement including a timing device having a definite operating time, control means for efiecting the operation of said timing means only during a predetermined mag-' nitude and phase range of the armature current of said machine relative to the armature voltage of said machine, a. second timing device having an operating time substantially equal to half the operating time of said first timing means, means controlled by said first timing device and said control means for effecting the timing operation of said second timing device only during said predetermined magnitude and phase range of armature current after said first timing device has completed its timing operation, and means 10. In a system including an alternating curof said machine relative to the armature voltage of said machine, a second'timing device having an operating time substantially equal to half the operating time of said first timing means, means controlled by said first timing device and said control means for effecting the timing opera- 1 tion of said second timing deviceonly durin said predetermined magnitude and phase range of armature current after: said'first timingdevice has completed its timing operation, and

' means responsive to said second timing means completing its timing operation for effecting a change in the field connections of said machine.

13. In a system including an alternating current supply circuit connected to an unexcited subsynchronously operating synchronous ma chine, speed responsive means dependent upon the motor armature current remaining within a predetermined magnitude and phase range relative to the motor armature voltage for a predetermined time, means for varying the minimum speed at which said speed responsive means responds, and means controlled by said speed responsive means for eifecting a predetermined change in the connections of said machine at substantially the same predetermined points in the slip cycle whenever said machine is operating at the minimum speed for which said speed responsive means is adjusted to respond. 14,111 a system including an alternatin rent supply circuit connected to an unexcited subsynchronously operating synchronous ma-- chine, speed responsive means dependent upon the motor armature current remaining within a predetermined magnitude and phase range relative to the motor armature voltage for a predetermined time, means ior varying the minimum speed at which said speed responsive means responds, and means controlled by said speed responsive means for eflecting a predetermined change in the field connections oisaid machine at substantially the same predetermined points in the slip cycle whenever said machine is op erating at the minimum speed for which said speed responsive means is adjusted to respond.

15. In a system including an alternating current supply circuit connected to an unexciied subsynchronously operating synchronous machine, speed responsive means dependent upon the motor armature current remaining within a predetermined magnitude and phase range relative to the motor armature voltage for a predetermined time, means for varying the minimum speed at which said speed responsive means responds, and means controlled by said speed responsive means for eflecting the application of excitation to the field winding of said machine at substantially the same predetermined points in the slip cycle whenever said machine is operating at the minimum speed for which said spced responsive means is adjusted to respond.

16. In a system including an alternating current supply circuit connected to a subsynchronously operating synchronous machine, speed responsive means dependent upon the impedance of the armature circuit of said machine, means for varying the minimum speed at which said speed responsive means responds, and means controlled by said speed responsive means for effecting a predetermined change in the connections 01' said machine at substantially the same predetermined points in the slip cycle whenever said machine is operating at the minimum speed for which said speed responsive means is ad Justed to respond.

1?. In a system including an alternating current supply circuit connected to an unexcited subsynchronously operating synchronous ma-- chine, speed responsive means dependent upon the impedance of the armature circuit of said machine, means for varying the minimum speed at which said speed responsive means responds, and means controlled by said speed responsive means for effecting the application of excitation to the field winding of said machine at points in the slip cycle favorable for synchronizing said machine whenever said machine is operating at the minimum speed for which said speed responsive means is adjusted to respond.

MAYNORD N. HALBERG.

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